Recording method for a phase-change optical recording medium

ABSTRACT

The present invention provides a recording method for a phase-change optical recording medium. The recording method of the present invention contains the step of irradiating an electromagnetic wave having a multipulse pattern so as to perform recording on a phase-change optical recording medium containing a phase-change recording layer. This method is characterised in that a starting time of a front pulse of the multipulse pattern delays 0.5 T to 1.25 T from a starting point of the first reference clock relative to the recording mark, where T is a reference clock of the multipulse pattern.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a recording method of a phase-changeoptical disk, namely, a rewritable optical disk. This method is suitablyapplied for a high volume optical medium, DVD+RW, or the like.

2. Description of the Related Art

Phase-change recording media, which are used for CD and DVD rewritablerecording media have rapidly become very popular due to their highcapacity, high-speed recording, and high compatibility with ROM (ReadOnly Memory). In recent years, it is required that recording/reproducingof mass image data is carried out at high speed, and higher speeds arebeing demanded of phase-change recording media. However, it is desirablethat a high linear velocity recording disk which can be recorded at ahigh linear velocity, should be able to be recorded also in a low-speeddrive for low linear velocity recording disks which record at a lowlinear velocity. This is possible with CD-R, which can record over awide range of linear velocity.

However, in the case of the above-mentioned phase-change recordingmedium, it is difficult to perform recording over a wide range of linearvelocity. In order to perform recording at a high linear velocity, ahigh power laser which outputs a high recording power is required. Therecording power of the laser light used in a low-speed drive is usuallylower than the recording power of the laser light output in a high-speeddrive for high linear velocity recording. Hence, it is difficult torecord a high linear velocity recording disk in a low-speed drive.

The aforesaid phase-change recording medium is usually optimized forrecording at a high linear velocity. In the case of a phase-changerecording medium designed in this way, the recording power to record ishigher than the optimal recording power for a low recording linearvelocity. Thus, in order to perform recording at a lower recording powerwith this phase-change optical recording medium, the sensitivity of thisphase-change optical recording medium must be increased. In order toincrease sensitivity of this phase-change optical recording medium, thereflectance of this phase-change optical recording medium can belowered. However, when designing this phase-change optical recordingmedium as a DVD, it is necessary to maintain compatibility with DVD-ROM.Thus, the above-mentioned reflectance cannot be made lower as desired.

The highest recording linear velocity in rewritable DVD currentlycommercialized in the past several years is 2.4×. A phase-change opticalrecording medium, which can be recorded at a higher recording linearvelocity than 2.4× and also in a low-speed drive, namely, which isdownward compatible, has not yet been provided.

In order to provide downward compatibility, it is required to select thecomposition of the above-mentioned phase-change optical recording mediumand the material of the recording layer, and optimize the recordingconditions of this phase-change optical recording medium, so that it isrecordable at a low recording power and the recording power margin islarge.

In the prior art, for example in Japanese Patent (JP-B) No. 3124720 orJapanese Patent Application Laid-Open UP-A) No. 2000-322740, bycontrolling the pulse-width of a laser pulse-like waveform, it can bemade CAV (Constant Angular Velocity) recording possible at 2.4×. In thecase of rewritable DVD, however, there is a problem that it is difficultto realize a recording linear velocity higher than 2.4× together withdownward compatibility so that recording can also be performed in alow-speed drive.

In JP-B No. 2844996, for example, instead of using a fixed erasing powerfor high speed recording, a method of modulating the erasing power bythe reproduction power range is disclosed. However, in the case of thismethod, a sufficient erasure cannot be performed, and there is theproblem that an amorphous phase may be formed depending on the level ofthe erasing power.

Also, for example, in JP-B No. No. 2941703, a method wherein a rear edgecooling pulse interval is basically eliminated when forming a recordmark, is described. However, in the case of this method, there is aproblem that it is difficult to form a record mark of predeterminedlength.

In the case of DVD, such phase-change optical recording medium andrecording method thereof are required that recording can be performed ata recording linear velocity as fast as 4× (as 1× linear velocity is 3.49m/s, this is approx. 14 m/s (13.96 m/s)), and also with a recordingpower below the optimal recording power of a phase-change recordingmedium for recording of a recording linear velocity of 1× to 2.4×.

In this regard, it is important to optimize the crystallization rate ofthe recording layer in the phase-change optical recording medium. In therecording layer, the overwrite characteristics, particular thecharacteristics of the first overwrite, deteriorate when recording isperformed at a high linear velocity. Hence, to enable recording at ahigh linear velocity, it is important to optimize the elements andelemental composition of this recording layer so that thecrystallization rate in this recording layer increases.

In order to form a record mark (amorphous phase) in the aforesaidrecording layer in the case of the aforesaid phase-change opticalrecording medium, it is necessary to heat the material of this recordinglayer to near the melting point thereof and to perform quenching in ashort time. The crystallization rate in the recording layer is larger,the larger is the temperature gradient over time, and the longer is thenon-heating time (cooling time) required to suppress recrystallizationis longer. However there is a limit to the heating time and coolingtime. During recording at a high linear velocity where it is difficultto raise the temperature in a short time, therefore, the recording powermust be increased. Also when recording at a low linear velocity, theaforesaid crystallization rate in the aforesaid recording layer islarge, so the recording power must likewise be increased.

Accordingly, in a phase-change optical recording medium for recording ata high linear velocity, the aforesaid crystallization rate cannot bemade too high. Consequently, if a fixed erasing power is irradiated, anamorphous phase is easily formed even if the erasing power is not sohigh, and the erasing power cannot be increased too much, the higher isthe linear velocity. For this reason, the above-mentionedcrystallization rate may be optimized at an intermediate linear velocitybetween a low linear velocity and a high linear velocity. However, inthis case, the erasing power is not sufficient for a high linearvelocity, and mark erasure properties during overwrite are poor.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention, which resolves theproblems inherent in the prior art, to provide a recording methodsuitable for a phase-change optical recording medium which can record ata high linear velocity, and which when recording at a low linearvelocity, can record at almost the same recording power as aphase-change optical recording medium suitable for a low linearvelocity. It further aims to provide a recording method which providesgood overwrite characteristics in CAV recording or CLV(Constant LinearVelocity) recording.

The first aspect of the recording method for a phase-change opticalrecording medium of the present invention comprises the step ofirradiating an electromagnetic wave having a multipulse pattern so as toperform recording, erasing and overwriting on a phase-change opticalrecording medium containing a phase-change recording layer, in which themultipulse pattern contains pulses of a peak power (Pp), an erasingpower (Pe) and a bias power (Pb), where the pulses of the peak powercontain a front heating pulse (OP1), an intermediate heating pulse (OPj)and a rear heating pulse (OPm), and the pulses of the bias power containa front cooling pulse (FP1), an intermediate cooling pulse (FPj) and arear cooling pulse (FPm). Here, a starting time of a front pulse delays0.5 T to 1.25 T (T: a reference clock of the multipulse pattern) from astarting point of the first reference clock relative to the recordingmark. In this recording method, several aspects are preferred: (1) anending time of a rear pulse is T-OPm or less, earlier than an endingpoint of the last reference clock relative to the recording mark; (2)the starting time of the front pulse delays more than 1 T, and 1.25 T orless, from the starting point of the first reference clock, with amaximum recording linear velocity among recordable recording linearvelocities to the phase-change optical recording medium; (3) the endingtime of the rear pulse is T-OPm earlier than an ending point of the lastreference clock, with the maximum recording linear velocity; (4)recording is performed at a recording linear velocity within a range ofan intermediate recording linear velocity to the maximum recordinglinear velocity with respect to the recordable recording linear velocityto the phase-change optical recording medium.

In the second aspect of the recording method of the present invention,when a recording linear velocity is continuously changes with respect toan inner circumference to an outer circumference of the phase-changeoptical recording medium, pulse widths of the front heating pulse (OP1),the intermediate heating pulse (OPj), and the rear heating pulse (OPm)are controlled by adjusting a sum of a time which is proportional to thereference clock relative to a recording linear velocity within a rangeof the minimum recording linear velocity and the intermediate recordinglinear velocity and a time being independent from the reference clock,or, a time which is proportional to the reference clock relative to arecording linear velocity within a range of one-third of the maximumlinear velocity to the maximum linear velocity. In this recordingmethod, if the aforesaid pulse widths are controlled by adjusting thesum of the time which is proportional to the reference clock relative toa recording linear velocity within a range of the minimum recordinglinear velocity and the intermediate recording linear velocity and thetime being independent from the reference clock, the method ispreferably applied with a recording linear velocity within the range ofthe minimum recording linear velocity to the intermediate recordinglinear velocity, and a recording linear velocity within the range of theone-third of the maximum recording linear velocity to the maximumrecording linear velocity.

In the third aspect of the optimal recording method of the presentinvention, in a case of recording only with the maximum recording linearvelocity regardless to a recording position on the phase-change opticalrecording medium, the aforesaid pulse widths are controlled by adjustinga sum of the time which is proportional to the reference clock and thetime being independent from the reference clock. Here, the timeindependent from the reference clock is 0.5 nano seconds or more.

In the fourth aspect of the recording method of the present invention,the multipulse pattern further contains at least one compensation pulsewhich includes a pulse of the erasing power Pe and a pulse of a seconderasing power Pe2 after the rear cooling pulse. Here, the second erasingpower Pe2 is higher than the erasing power Pe. In this aspect of therecording method, a few aspects are preferred: (1) the compensationpulse is contained in the multipulse pattern when at least the shortestrecord mark is recorded among recordable record marks with thephase-change optical recording medium and the recording linear velocity;and (2) at least the maximum recording linear velocity is applied so asto perform recording.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an example of the laminarstructure of the phase-change optical recording medium used by thepresent invention.

FIG. 2 is a diagram showing an example of a light emission waveform usedto perform recording and erasure of the prior art.

FIG. 3 is a diagram showing an example of a light emission waveform usedto perform recording and erasure according to the present invention.

FIG. 4 is a graph showing the dTtop dependency of jitter after oneoverwrite at a linear velocity of 14 m/s.

FIG. 5 is a graph showing the relation between jitter and dTera.

FIG. 6 is a graph showing the relation between jitter and power margin.

FIG. 7 is another graph showing the relation between jitter and powermargin.

FIG. 8 is a graph showing the dTtop dependencies of jitter with respectto each linear velocity at the first overwriting.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, the phase-change optical recording medium used bythe present invention is formed by laminating a transparent substrate 1,lower dielectric protective layer 2, phase-change recording layer 3which undergoes a reversible phase-change between an amorphous phase anda crystalline phase, an interface layer 7, upper dielectric protectivelayer 4, anti-sulfuration layer 5 and reflective layer 6, in that order.The interface layer 7 is not indispensable.

For the aforesaid transparent substrate 1, plastic or glass, such astransparent polycarbonate (PC), polymethacrylic acid (PMMA) or the like,which is transparent to the wavelength of the recording/reproducinglight, may be used.

There is no particular limitation on the material of the lowerdielectric protective layer 2 disposed between the transparent substrate1 and the phase-change recording layer 3, and the material of the upperdielectric protective layer 4 disposed between the phase-changerecording layer 3 and the reflective layer 6. Although they may beselected according to the purpose, examples are metal oxides such asSiO_(x), ZnO, SnO₂, Al₂O₃, TiO₂, In₂O₃, MgO, ZrO₂, Ta₂O₅, or the like;nitrides such as Si₃N₄, AlN, TiN, BN, ZrN, or the like; sulfides such asZnS, TaS₄, or the like; and carbides such as SiC, TaC, B₄C, WC, TiC, Zror the like.

These materials can also be used alone or in admixture. Of these, amixture of ZnS and SiO₂ is generally used as the phase-change recordingmedium. As the mixing ratio thereof, 80:20 (molar ratio) issatisfactory. The aforesaid lower dielectric protective layer 2preferably has a low thermal conductivity, and its specific heat issmall. It is preferred that crystallization is not caused byoverwriting, cracking is not occurred by repeated heating/quenchingcycles, and there is no elemental diffusion. ZnS—SiO₂ (80:20) satisfiesthese conditions, and is used also for the upper dielectric protectivelayer 4. In mixtures wherein ZrO₂ contains 3 mol % to 6 mol % of Y₂O₃,the refractive index is almost the same as or is larger than that ofZnS—SiO₂, and its thermal conductivity is also low.

When the bulk thermal conductivity was measured by the laser flashprocess, in a type of the bulk containing ZrO₂ as the main ingredient,ZrO₂.Y₂O₃ (3 mol %), ZrO₂.SiO₂ (5 mol %) Y₂O₃ (3 mol %), ZrO₂.TiO₂ (50mol %).Y₂O₃ (3 mol %), and ZrO₂.TiO₂ (40 mol %).SiO₂ (20 at %).Y₂O₃ (3mol %) were respectively 5.1, 3.5, 1.73, 2.6 (W/m.K), and ZnS.SiO₂ (20mol %) was 8.4 (W/m.K).

The refractive index (n) was 2 or more in all cases except for ZrO₂.SiO₂(5 mol %). MgO may be used instead of Y₂O₃. All these materials aregenerally used for preventing cracking of the target when a target ismanufactured for film-forming by a sputtering method.

For references, phase-change optical recording mediums were formed usingthese materials for the upper dielectric protective layer 4, and thestorage properties of record mark thereon were examined at 80° C. and85% RH after recording. If the content of ZrO₂ is 50 at % or more, themark disappeared or jitter deterioration was large. However, the repeatoverwrite characteristics of ZrO₂ containing materials are good, andthere was less jitter degradation after recording 1,000 times than withZnS.SiO₂. There is more effect in overwriting at a high linear velocity.

However, as the aforesaid dielectric protective layer 4, ZnS—SiO₂(80:20) is more suitable.

In this regard, in order to harness the effect which improves thisoverwrite characteristic, the effect of providing a ZrO₂ material as aninterface layer disposed between the phase-change recording layer andthe upper dielectric protective layer was examined.

As a result, it was found that when the thickness was within a range oflnm to 5 nm, this effect was maintained and deterioration of storagereliability was considerably suppressed.

The interface layer has the following effect. This layer is in acrystalline state and the lattice constants thereof is close to that ofthe aforesaid phase-change recording layer so that this interface layerencourages crystal growth of the phase-change recording layer. Althoughthe interface layer is not in a crystallized state, it assists crystalgrowth so as to increase the erasure ratio and improve overwritecharacteristics.

Moreover, as wettability is poor, when the recording layer is in amolten state, fluidity is suppressed and local volume change of thisrecording layer is suppressed, overwrite characteristics also improve.

The thickness of the lower dielectric protective layer is within a rangeof about 40 nm to about 250 nm, and preferably within a range of 45 nmto 80 nm. If the thickness is less than about 40 nm, environment-proofprotective functions are reduced, the heat dissipation effect is loweredso that deterioration of repeat overwrite characteristics increases. Ifthe thickness is more than about 250 nm, in the film-forming process bysputtering or the like, film peeling or cracking occurs due to the riseof film temperature.

Moreover, the thickness of the transparent substrate is less than 0.6mm. If the thickness of the transparent substrate reaches 0.6 mm,deformation of the transparent substrate may increase, and aftersticking, it may be impossible to correct the deformation.

The thickness of the upper dielectric protective layer is in a range ofabout 5 nm to about 50 nm, and preferably in a range of 8 nm to 20 nm.If the thickness is less than about 5 nm, recording sensitivity willfall, whereas if the thickness is more than about 50 nm, deformationoccurs due to temperature rise, and repeat overwrite characteristicswill worsen due to lower heat dissipation properties.

The aforesaid reflective layer may comprise a metallic material, such asAl, Ag, Cu, Pd, Cr, Ta, Ti and the like. The thickness thereof ispreferably within a range of 50 nm to 250 nm. If the reflective layer isexcessively thick, heat dissipation properties are more improved, butdue to temperature rise of the medium while producing the thin film,deformation of the substrate does occur. If the reflective layer isexcessively thin, heat dissipation properties will worsen and recordingproperties will deteriorate.

The characteristics of the above-mentioned reflective layer are improvedby using Ag which has a higher thermal conductivity. For this reason, Agor Ag alloy is suitably used.

When the linear velocity increases, the cooling rate will become large.Accordingly, an amorphous mark is easily formed, but as the recordinglayer is heated to near its melting point when the mark is formed, theheating pulse time of the light emission pulse had to be lengthened. Onthe other hand, if the heating time is lengthened, the cooling time isshortened so the cooling time may not be sufficient and it will becomedifficult to form a mark. This is because the sum of the pulse times ofone heating and cooling is a reference clock, and changes are madewithin these limitations.

Thus, in order to improve the cooling efficiency of the medium, Ag maybe suitably used. However, when the upper dielectric protective layercontains S (sulfur) and the reflective layer is Ag, Ag₂S is easilyformed under high temperature and high humidity. This leads tocharacteristic degradation and generates defects, which poses a problem.

It is then necessary to provide an anti-sulfuration reaction layerbetween the reflective layer and the upper dielectric protective layer.As a result of intensive studies on oxides, nitrides, carbides andmetals performed so far, preferable materials for the anti-sulfurationreaction layer are Si and SiC, and ZrO₂, MgO and TiO_(x) are alsosuitable. SiC prevents the reaction of Ag and S, and the effect thereofis large even with the thickness of as thin as 3 nm. The thickness ofthe anti-sulfuration reaction layer is within a range of about 2 nm toabout 10 nm. If the thickness is more than 10 nm, it will separate fromthe reflective layer, so the heat dissipation efficiency falls and asthe absorption constant is high, and the reflectance tends to fall.

By using Ag alone for the reflective layer, properties improve. In viewof the corrosivity of Ag itself and adhesion to the anti-sulfurationlayer when Ag is used alone, by optimizing sputtering conditions (argongas pressure) during thin film production, the crystal particle size ofAg is reduced, and by suppressing particle growth, the thin film surfaceof Ag becomes flat and smooth. When the particle size is large, peelingeasily occurs from where the adhesion is weak.

Further, to improve adhesion, Ag can still be used alone by optimizingthe curing conditions and thickness of the ultraviolet-curing acrylicresin which is used as an environmental protection layer on thereflective layer. If Ag is used alone, however, there is a concern thatdeterioration may occur due to manufacture under less than optimumconditions, to storage conditions of the substrate before stickingwithout a recording film, to moisture absorption of the substrate itselfand moisture absorption of the ultraviolet curing resin.

In this regard, reliability is improved by using an alloy containing 95at % or more Ag. If the addition amount of other metallic element to Agexceeds 5 at %, thermal conductivity considerably decreases. For thisreason, the addition amount is preferably 2 at % or less.

As additional elements, Cu and Ni are preferably since they suppressparticle size growth without much lowering thermal conductivity andimprove environmental resistance. When producing an Ag film bysputtering, to reduce the crystal particle size of the Ag film, thepower applied between the substrate and the target may be 3 W or less.

The aforesaid phase-change recording layer has conventionally been basedon a eutectic composition in the vicinity of Sb70Te30. AgInSbTe andAgInSbTeGe materials which contains Ag, In and Ge are suitable forhigh-density recording at high linear velocity, and have thereforeconventionally been used. The higher is the ratio of Sb to Te or if theamount of Sb is more than 80 at %, the crystallization rate increases,but storage properties are exceedingly poor and it becomes difficult toform an amorphous phase. Therefore, a desirable amount of Sb for highlinear velocity recording is within a range of 65 at % to 80 at %.

On the other hand, the amount of Te is preferably within a range of 15at % to 25 at %. Although Ge does not increase the crystallization rate,Ge improves storage properties of the record mark under the hightemperature environment and is an essential element. The binding energyof Ge and Te is large. Moreover, the larger is the Ge addition amount,the higher is the crystallization temperature which means better storageproperties. However, if Ge is excessively added, the crystallizationtemperature further increases and the crystallization rate slows down,so 5 at % is preferred. Although Ag stabilizes record marks, thecrystallization temperature can not sufficiently increase thecrystallization temperature. If Ag is excessively added, thecrystallization rate drops, so a large amount thereof cannot be used. Onthe other hand, it also has the effect of stabilizing the crystallinestate, so the amount of Ag in the phase-change recording layer ispreferably 3 at % or less.

In increases crystallization rate and increases crystallizationtemperature, so storage properties are improved. If a large amount isadded, however, the material tends to segregate. Deterioration of repeatoverwrite characteristics and deterioration with respect to reproductionoptical power occur, so the amount of In in the phase-change recordinglayer is preferably 5 at % or less. In addition to In, Ga also increasesthe crystallization rate. Ga accelerates the crystallization rate morethan an identical amount of In, but it also increases thecrystallization temperature. If the amount of Ge is 5 at % and Ga is 5at % or more, the crystallization temperature will greatly exceed 200°C. and may rise to 250° C. or more. As a result, in the initializationprocess for crystallizing the recording layer from the amorphous state,the reflectance distribution around the track increases, and leads torecording characteristic and data errors. For this reason, when Ga isused as a supplementary element to accelerate the crystallization rate,the amount of Ga in the phase-change recording layer is preferably 3 at% addition or less.

There is a limit to the use of AgInSbTeGe as a higher linear speedmaterial. As a result of studying elements which replace Ag and In, itwas found that although Mn accelerates the crystallization rate, it iseffective as it does not increase the temperature rise more thannecessary. Mn increases crystallization rate, like In. Even if a largeamount thereof is used, storage properties are satisfactory withoutdeteriorating overwrite characteristics. Although crystallizationtemperature also increases, the increase amount in the crystallizationtemperature relative to the amount of Mn is small, and reproductivephotodegradation is also small. In the present invention, it issufficient if 5 at % of Mn is added.

Thus, a GeMnSbTe material is also a suitable material for high linearvelocity. Further, a GeMnSbTe material wherein Ga is added to improvecrystallization rate and storage properties is also effective. Thethickness of the recording layer, is preferably within a range of 10 nmto 20 nm. If the thickness thereof is less than 10 nm, the reflectancedifference between the crystal phase and amorphous phase is small. Ifthe thickness thereof is more than 20 nm, recording sensitivity andrepeat overwrite characteristics will worsen.

As a material for the phase-change recording layer, apart from theaforesaid materials, Ag.In.Sb.Te, Ge.Ga.Sb.Te, Ge.Sb.Te, Ge.Sn.Sb.Te,Ge.Sn.Sb, Ge.In.Sb.Te, Ga.Sn.Sb, Ge.Ag.Sn.Sb, Ga.Mn.Sb, Ga.Sn.Sb.Se, andthe like can also be used.

Recording/reproducing for the above phase-change recording medium can beperformed with the recording wavelength of 400 nm to 780 nm.

In the case of DVD, a recording wavelength of 650 nm to 660 nm is used.The numerical aperture of the object lens is then set to 0.60 to 0.65,and the beam diameter of the incident light is set to 1 μm or less.Hence, the thickness of the substrate is set to 0.6 mm, and theaberration is made small.

The pitch between the grooves in which the mark is written is 0.74 μm,the depth of the grooves is 15 nm to 45 nm and the groove width is 0.2μm to 0.3 μm.

The groove has a wobble having a frequency of approximately 820 kHz. Theaddress part is encoded in the wobble by modulating the phase of thefrequency. This phase-change is detected and decoded to a binary signal,and an address (number) is read.

The amplitude of this wobble is 5 nm to 20 nm. The recording lineardensity is 0.267 μm/bit, and recording is performed using the (8-16)modulation method. In this case, the shortest mark length will be 0.4μm. 2× of DVD is recorded at a linear velocity of 7 m/s (6.98 m/s) andwith the reference clock whose frequency is set to 52.3 MHz (T:19.1nanoseconds, T is a reference clock). 4× of DVD is recorded at a linearvelocity of 14 m/s (13.96 m/s), and with the reference clock whosefrequency is set to 104.6 MHz (T:9.56 nanoseconds). The linear velocityis changed from 1× to 4× while irradiating an erasing power of fixedmagnitude continuously or at regular intervals to the phase-changeoptical recording medium. If the reflective signal strength at this timeis measured, the reflective intensity begins to decrease from a certainlinear velocity, and at a higher linear velocity, the reflectiveintensity further decreases, and eventually becomes saturated.

When a substrate surface of the phase-change optical recording medium ismeasured using a pickup of wavelength 659 nm and NA 0.65, and a 12 mWerasing power is irradiated, the linear velocity at which reflectancebegins to fall is from 9 m/s to 10.5 m/s.

Conventionally, a phase-change optical recording medium optimized at 4×requires a higher recording power than a recording power required for amedium optimized at a lower linear velocity. In order to make recordingpossible by the same recording power as a phase-change optical recordingmedium corresponding to from 1× to 2.4×, the linear velocity at whichthe reflectance begins to fall is preferably earlier than 2.4×, i.e.,8.4 m/s, and more preferably 0.5 m/s to 1 m/s.

FIG. 2 is a light emission waveform used conventionally to performrecording/erasing. As irradiation powers, there are peak power (Pp),erasing power (Pe) and bias power (Pb). As a pulse pattern, there are afront heating pulse OP1, an intermediate heating pulse OPj (j=2 to m-1),and a rear heating pulse OPm, at which the peak power (recording power)is applied, for heating the recording layer. In addition, there arecooling pulses of a front cooling pulse FP1 and an intermediate coolingpulse FPj. Here, the sum of the time of the intermediate heating pulseOPj and the intermediate cooling pulse FPj, is T.

The number of pulses is (n-1) or (n-2) relative to the record marklength nT. Up to a linear velocity of 2.4×, a record mark ofpredetermined length can be recorded while adjusting Δ2=0, Δ1 to amaximum of 0.5*T, and Δ3 to from 0 T to 0.5 T. Up to a linear velocityof 2.4×, good recording characteristics are obtained. However, therecording linear velocity becomes higher and the linear velocity reaches4× (14 m/s), according to this method, it becomes more difficult toacquire sufficient overwrite characteristics.

In the case of the aforesaid phase-change optical recording medium, at alinear velocity of 4×, the more the erasing power is increased, and thelonger a rear cooling pulse FPm is lengthened, hence the worse are thefirst overwrite characteristics. This means that the erasure rate ofprevious record marks is worse. This is because the mark length of theamorphous phase region of the record mark rear edge widens, and the marklength increases. As the optimal range of erasing power is narrow, therecrystallization rate is slower. In other words, the recording layer issufficiently heated, and the velocity is reduced to grow the crystals ata lower temperature than the fastest speed at which crystals grow fromthe molten state.

For this purpose, in the present invention, at least at the maximumrecording velocity, by completing the ending time of the rear pulseT-OPm earlier than the record mark ending part, overwritecharacteristics can be improved. In other words, it is effective to makethe rear cooling pulse width zero, or to shorten it as much as possible.

In the present invention, moreover, at least at one recording linearvelocity, by starting a front pulse, i.e., a front pulse of the peakpower 0.5 T to 1.25 T later than a starting point of the first referenceclock relative to the mark, jitter can be suitably controlled at a lowlevel.

It is also effective to apply these conditions to a range from theintermediate linear velocity to the maximum linear velocity amongrecordable linear velocities ranges to the phase-change opticalrecording medium. This means assigning a maximum width of T-Pm to“dTera” relative to the position b of FIG. 3.

Here, the intermediate recording linear velocity is 2.4× whichcorresponds to 3.49×2.4 m/s and the maximum recording linear velocity is4× which corresponds to 3.49×4 m/s in that case.

In FIG. 3, “dTop” is a variable range of a starting time of a frontheating pulse relative to “a” (a: the position which is 1 T delayed fromthe starting point of the first reference clock relative to therecording mark). If the front heating pulse starts earlier than position“a”, it is assigned (+) and if it starts later, it is assigned (−).Therefore, if it is 0.5 T to 1.25 T delayed from the starting point ofthe first reference clock relative to the recording mark, “dTop” lieswithin a variable range of −0.25 to +0.5 T. “OP” is the irradiation timeof the peak power Pp (heating pulse). “OP1” is the irradiation time ofPp of the front pulse. “OPj (j=2−(m-1))” is the irradiation time of Ppof the intermediate pulse. “OPm” is the irradiation time of Pp of therear pulse. “FP” is the irradiation time of the bias power Pb (coolingpulse). “FP1” is the irradiation time of Pb of the front pulse. “FPj(j=2−(m-1))” is the irradiation time of Pb of the intermediate pulse.“FPm” is the irradiation time of Pb of the rear pulse. “dmp” is thevariable range of the starting time of the intermediate heating pulse.“dlp” is the variable range of the starting time of the rear heatingpulse. “dTera” is the variable range of the ending time of the rearcooling pulse relative to “b” of FIG. 3. If the rear cooling pulse endsearlier than position “b”, it is assigned (+) and if it ends later, itis assigned (−). “dint” is the time from the end position of the rearcooling pulse to the starting position of the compensation pulse. “dera”is the irradiation time of the second erasing power (Pe2). In theabove-mentioned FIG. 3, “Pe2” has is the same value as “Pe1”, and “dint”and “dera” are 0.

Subsequently, with Pe2>Pe1, Pe2 is a power for which, if continuousirradiation is performed at the recording linear velocity, thereflectance does not decrease compared to its value prior toirradiation. A compensation pulse which sets a time dint for irradiatingthe erasing power Pel and a time dera for irradiating Pe2 from theending time of the rear cooling pulse to optimal times, is added. Thismultipulse pattern may comprise one or more compensation pulses ifnecessary.

This compensation pulse may be applied to recording of all record marklengths or to shorter record mark lengths. In this case, record marklength is preferably 3 T, 4 T, and 5 T. All of these, 3 T alone, or 3 Tand 4 T are preferable (there is no case of 3 T, 5 T, or 5 T only). Inparticular, although the shortest record mark length is 3 T in DVD, itmay be applied only when recording is performed to form record marklength of 3 T, 4 T or 5 T. These compensation pulses eliminate dataremaining after erasing in a course of overwriting. For this reason,they are required in order to promote recrystallization. In addition tothe present purpose, this compensation pulse becomes more effective asthe recording linear velocity increases. The higher is the recordinglinear velocity, the more time is required from when the erasing poweris irradiated to raise the temperature of the recording layer to thetemperature of the molten state.

However, if it is attempted to increase the erasing power Pe1, as thispower is irradiated continuously or at regular intervals until the nextmark is recorded, the recrystallization region widens, or due to thehigher linear velocity together with the quenching effect, the amorphousphase region widens. Hence, by providing the compensation pulse, controlof the rear edge of the record mark is easier. The optimal times fordera and dint are 0.2 T<dera<3 T and 0<dint<1 T respectively. As aresult, the erasing rate after the first overwriting are improved, andjitter characteristics improve. The range of each heating pulse widthOPk (k=1, . . . m) is 0.2 T to 0.8 T. In the case of DVD, when it isapplied to from 1× to 4× and recording is made by CAV from 1× to 2.4×,the reference clock corresponding to each linear velocity variescontinuously, but an optimal recording is attained by adjusting eachheating pulse width by the sum of a time proportional to the referenceclock T and a fixed time independent from the reference clock.

Specifically, this is (1/a) T*i+b*j (a, b, i, j are integers[nanoseconds]).

For recording at 4×, a=16, and to increase control time resolution, thepulse width is set to T*i ( 1/16), which is mainly used in CLV. Tocontrol from 1.7× to 4× by CAV, a=16 and b=1, and ( 1/16) T*i+1*j isused.

The sum of the front heating pulse and the front of cooling pulse, thesum of the intermediate heating pulse and the intermediate coolingpulse, and the sum of the rear heating pulse and the rear cooling pulseare basically 1 T, but the sum of the front heating pulse and the heatof cooling pulse and the sum of the rear heating pulse and the rearcooling pulse are not limited thereto.

By adjusting the aforesaid sums within a range from 0.3 T to 1.5 T, arecord mark of predetermined length can be recorded.

In CAV recording from 1× to 2.4×, the pulse width is adjusted byT*i(⅙)+2*j.

EXAMPLES

Hereafter, the method of the present invention will be describedreferring to specific examples.

Example 1

A phase-change optical recording medium was prepared as follows.

As a transparent substrate in which record marks are formed in thegroove thereon, a polycarbonate substrate having a groove pitch of 0.74μm, a groove width of 0.25 μm, a groove depth of 25 nm and a thicknessof 0.6 mm was used. Each layers was formed on the transparent substrateby the sputtering method. Address information was provided in a wobbleof the groove having a frequency of 818 kHz, with 180 degrees reversedphase depending on the information.

The lower dielectric protective layer was formed with thickness of 69 nmon the aforesaid substrate using a target of ZnS:SiO₂=80:20 (mol %).Next, the phase-change recording layer which isGe:Ag:In:Sb:Te=3:0.8:3.5:72:20.7 was formed so that its thickness was 14nm. Next, the interface layer was formed so that its thickness was 2 nmusing a multiple oxide target of ZrO₂:TiO₂:Y₂O₃=49:45:6 (mol %). Next, aupper dielectric protective layer was formed to a thickness of llnmusing a target of ZnS:SiO₂=80:20 (mol %). A SiC layer of thickness 4 nmand Ag layer of thickness of 140 nm were formed thereupon. Next, toimprove environmental resistance, an ultraviolet curing resin (SD318,Dainippon Ink and Chemicals, Incorporated.) was applied and hardened soas to form an environmental protection layer having a thickness of 5 μm.Finally, the aforesaid transparent substrate was affixed to theenvironmental protection later via an ultraviolet curing resin layer(DVD 003 [acrylic], Nippon Kayaku Co., Ltd.) with a thickness of 40 μmto obtain a phase-change optical recording medium.

Even when this phase-change optical recording medium was subjected to atest at 80° C., and 85% RH, or a heat cycle test between 25° C. and 40°C. at 95% RH, defects did not occur. Next, the aforesaid recording layerwas crystallized using a large caliber LD of wavelength 810 nm (beamdiameter: track direction ltm x radial direction 75 μm) at a linearvelocity of 9 m/s, power of 900 mW and head feed rate of 18 μm/rotation.

When DC light of 12 mW was irradiated to the phase-change opticalrecording medium by the optical head equipping the LD with continuouslychanging the linear velocity, reflectance began to decrease from thevicinity of a linear velocity of 9.5 m/s. Recording and reproductionwere performed using a pickup head having a wavelength of 657 nm and anobject lens NA of 0.65, and recording was performed at a maximum linearvelocity of 14 m/s to give a recording density of 0.267 μm. The mode ofmodulating recording data was (8,16) modulation. Recording was performedso that the recording power was a maximum of 19 mW, the bias power was0.5 mW and the erasing power was 30% of the recording power. The numberof pulses of each mark length is (n-1) (n=3-14).

The conditions in the case of recording by CLV at a linear velocity of14 m/s (4×) and by CAV at a linear velocity of from 1× to 2.4× are shownin Table 1. Herein, “dTop” is written as “dTtop”, “OP1” is written as“Ttop”, “Opj”, and “OPm” are written as “Tmp”. Also, “dmp”, “dip”,“dint” and “dera” were all set to zero. These conditions are based onthe method of FIG. 3.

In addition, “dTtop” shown in the table 1 was measured based from “a” ofFIG. 3. When the starting time of the front pulse is earlier than “a” itis assigned (+) and if it starts later, it is assigned (−). Looking atthe starting position of record mark (the time T earlier than a), “a”corresponds “dTtop=1 T”. “−0.25 T” refers to the position 1.25 T apartfrom the record mark starting position, and “0.5 T” refers to theposition 0.5 T apart from the recording marl starting position. Here,the record mark starting position corresponds to a starting point of thefirst reference clock. TABLE 1 Linear velocity 14 m/s 8.4 m/s 35 m/sReference clock: T 9.55 nsec. 15.67 nsec. 38.22 nsec. Parameters (nsec)dTtop −( 2/16) * T ( 2/6) * T Ttop ( 5/16) * T + 2 ( 2/6) * T + 6 Tmp (6/16) * T + 2 (⅙) * T + 4 Tmp ( 6/16) * T + 2 (⅙) * T + 4 dTera (6/16) * T (⅙) * T

The “dTtop” dependency of jitter after one overwriting at a linearvelocity of 14 m/s is shown in FIG. 4. The recording power is 17 mW. Bydelaying the starting position, the jitter margin widens. In the priorart, “dTtop” of FIG. 4 was more than zero, and the jitter after oneoverwrite exceeded 9%. Even the case of 9% or less was within the rangeof 0 to 0.25 T, the margin was narrow. For this reason, “dTtop” must befinely controlled to approximately 1/16 of the reference clock.

From FIG. 5, it is seen that the earlier the ending time of the coolingpulse at the rear edge (+side) is finished, the better are thecharacteristics. The power margin of the jitter during recording at arecording linear velocity of 2.4× and 4× for recording under theconditions of Table 1 which reflects these conditions, is shown in FIGS.6 and 7. A 4× recording power margin is guaranteed, and for 2.4×, thereis a large margin under a recording power of 15 mW, so 4× recording ispossible and there is downward compatibility.

Example 2

Using the same phase-change optical recording medium as in Example 1, asshown in Table 2, in the case of 4× recording, the pulse width wasadjusted by a time proportional to a reference clock, and “dTera” wastaken as T-Tmp. In the case of CAV recording at an intermediate linearvelocity which is corresponded to a range of 8.4 m/s to 3.5 m/s when themaximum linear velocity is 14 m/s and minimum linear velocity is 3.5m/s, as shown in Table 2, the pulse width was adjusted by a timeproportional to the reference clock and a fixed time.

As a result, for a linear velocity of 14 m/s, recording was performed ata recording power of 17 mW and erasing power of 5.3 mW. For 8.4 m/s and3.5 m/s, recording was performed at a recording power of 15 mW anderasing power of 7.5 mW. In all cases, the jitter was 9% or less up to1,000 times of overwriting. TABLE 2 Linear velocity 14 m/s 8.4 m/s 35m/s Reference clock: T 9.55 nsec. 15.67 nsec. 38.22 nsec. Parameters(nsec) dTtop −( 2/16) * T ( 2/6) * T Ttop ( 9/16) * T ( 2/6) * T + 6 Tmp( 9/16) * T (⅙) * T + 4 Tmp ( 9/16) * T (⅙) * T + 4 dTera ( 7/16) * T(⅙) * T

Example 3

The same phase-change optical recording medium as in Example 1 was used,and the recording layer material was Ge:Ag:In:Sb:Te=2:0.5:3.5:72.5:21.5.The pulse width was adjusted by a time proportional to a recordinglinear velocity of 6 m/s exceeding 1/3 of the maximum linear velocity of14 m/s, and, a time which was proportional to the reference clock of 14m/s, and a fixed time, and CAV recording was performed. At 6 m/s and 8.4m/s, the recording power was 15 mW, at 14 m/s the recording power was 18mW, and the jitter was 9% in all cases. The recording conditions areshown in Table 3. TABLE 3 Linear velocity 14 m/s 6 m/s Reference clock:T 9.55 nsec. 22.22 nsec. Parameters (nsec) DTtop −( 2/16) * T ( 2/6) * TTtop ( 6/16) * T + 1.8 Tmp ( 6/16) * T + 1.8 Tmp ( 6/16) * T + 1.8 DTera( 6/16) * T 0

Example 4

The same phase-change optical recording medium as in Example 1 was used,and recording was performed at a recording linear velocity of 14 m/s andrecording power of 17 mW. The recording conditions are as shown in Table4. Each pulse width was adjusted to a time which was proportional to thereference clock. For record marks from 3 T to 14 T, the compensationpulse was applied only when recording the record mark of 3 T. Theerasing powers were Pe1=5.3 mW and Pe2=6.0 mW. The compensation pulsestarting time was set to dint=0 T and the compensation pulse irradiationtime (pulse width) was set to dera=0.5 T. As a result, when there is 9%jitter on the first overwriting without a compensation pulse, the jitterwas 8% with the compensation pulse. When there is 8% jitter after 1,000times of overwriting without the compensation pulse, the jitter was 7.5%with the compensation pulse after 1,000 times of overwriting. Therefore,there is a large effect in improving the first overwriting where highdensity and high linear velocity are a problem with a phase-changeoptical recording medium. TABLE 4 Linear velocity 14 m/s Referenceclock: T 9.55 nsec. Parameters (nsec) DTtop −( 2/16) * T Ttop ( 9/16) *T Tmp ( 9/16) * T Tmp ( 9/16) * T DTera ( 9/16) * T Dera ( 8/16) * T 3Tonly 0 4T-14T Pe1(mW) 5.3 Pe2(mW) 6

Example 5

The same phase-change optical recording medium as in Example 1 was used,and the recording condition was also the same as in Example 1 expectdTtop.

dTtop dependency of jitter relative to each recording liner velocity atthe first overwriting is shown in FIG. 8.

For the recording linear velocity of 14 m/s, recording was performed ata recording power of 19 mW and an erasing power of 5.7 mW. For therecording linear velocity of 3.5 m/s, recording was performed at arecording power of 16 mW and an erasing power of 8 mW. Especially withthe recording linear velocity of 14 m/s, the jitter exceeded 9% in theconventional range of dTtop which was 0 T to 0.5 T (the starting time ofthe front pulse starts 0.5 T to 1.0 T later than the starting point ofthe first reference clock). When dTtop is more than 0.5 T (the startingtime of the front pulse starts less than 0.5 T later than the startingpoint of the first reference clock), the jitter exceeded 9% with allrecording linear velocities.

Example 6

The same phase-change optical recording medium and the recording methodas in Example 1 were used expect that the material of the recordinglayer was changed to Ge:Ga:Sb:Te=4:2:73:21. Recording was performed at arecording linear velocity of 14 m/s, a recording power 18 mW, theerasing power 5.6 mW. As a result, the jitter remained 9% or less up to1,000 times of overwriting.

Example 7

The same phase-change optical recording medium and the recording methodas in Example 1 were used expect that the material of the recordinglayer was changed to Ge:Sn:Sb:Te=4.0:4.5:71.0:20.5. Recording wasperformed at a recording linear velocity of 14 m/s, a recording power 18mW, the erasing power 5.4 mW. As a result, the jitter remained 9% orless up to 1,000 times of overwriting.

According to the aforesaid first aspect, a recording method can beprovided in which there is downward compatibility, recordingcharacteristics are maintained even if there is downward compatibility,and recording characteristics are excellent even at a high linearvelocity.

According to the aforesaid second aspect, a recording method is providedwhich permits CAV recording within a predetermined range of recordinglinear velocity from the minimum recording linear velocity range to themaximum recording linear velocity range in which recording can beperformed on the phase-change optical recording medium.

According to the aforesaid third aspect, a recording method is providedwhich permits the recording property margin to be widened.

According to the aforesaid fourth aspect, a recording method is providedwhich has excellent overwrite characteristics at a high recording linearvelocity.

1-20. (canceled)
 21. An optical recording method, comprising the steps of: irradiating a pulsed electromagnetic wave on a phase-change optical recording medium, said recording medium having a phase-change recording layer to record a mark, wherein a waveform of the irradiated electromagnetic wave has a pulse shape having an irradiating power and an irradiating period; wherein the irradiating power of the pulsed electromagnetic wave has a peak power, erase power, and bias power; wherein the irradiating period of the pulsed electromagnetic wave has an irradiating period of the peak power and an irradiating period of the bias power; and wherein an irradiation of the pulsed electromagnetic wave as a rear heating pulse is followed by an irradiation of the pulsed electromagnetic wave of the erase power, irradiation of the pulsed electromagnetic wave as a rear heating pulse being a trailing pulse of the peak power in recording the mark.
 22. The optical recording method according to claim 21, wherein the irradiation of the pulsed electromagnetic wave as a rear heating pulse is followed by substantially no irradiation of a rear cooling pulse as a pulse of the bias power, the irradiation of the pulsed electromagnetic wave as a rear heating pulse being further followed by the irradiation of a pulsed electromagnetic wave of the erase power.
 23. The optical recording method according to claim 21, wherein a recording of a mark by irradiating the pulsed electromagnetic wave is performed at a recording linear velocity between an intermediate linear velocity and a maximum linear velocity. 